The invention generally relates to a composition and method for preventing and controlling bacterial infection in plants, and more particularly to preventing and controlling pathogenic bacterial infection in plants through the use of a copper-phosphite compound and/or with a nutrient-halo-phosphite compound.
Bacterial plant pathogens pose especially unique problems for disease control. The primary control strategy for bacterial diseases is based on exclusion of the pathogen through the use of disease free seed or propagative parts for initial planting of perennial plants or annual planting of field and vegetable crops, or quarantine and eradication if bacterial pathogens are introduced into an area. There are only a few chemical controls (antibiotics) for established bacterial diseases, and their use is limited because of phytotoxicity or pathogen mutations for resistance. Commonly applied protective copper compounds (for example sulfates or oxides) have limited benefit in controlling bacterial diseases because of their limited penetration into plant tissues where bacteria establish themselves, and mutations provide bacteria with resistance to these materials.
Unlike the control of disease outbreaks in annual crops that can be remediated in subsequent years through sanitation and the use of bacteria-free seed stocks, replanting of perennial crops such as citrus involves high capital costs to establish the planting, and several years after planting before production is initiated. Established bacterial diseases such as those caused by Candidatus Liberibacter species (citrus greening or Huanglongbing, psyllid yellows of tomato, or purple top and zebra chip of potatoes, etc.) that survive in alternate host plants in the environment and are disseminated by insect vectors (several species of psyllids) that commonly infect throughout the plant life cycle are very difficult to contain because of the wide dissemination range of the insect vector and long lag time for symptom expression (Bove, 2006).
Quarantine and eradication of infected plants can be as commercially damaging as the disease they are implemented to control. This was exemplified by the reintroduction of bacterial citrus canker (Xanthomonsas citri) to Florida in 1996 and the resulting eradication of almost 50% of commercial citrus production before the effort was abandoned in 2005 because this bacterial disease became established throughout the area by hurricanes before containment could be accomplished. Citrus canker quarantines and decontamination efforts currently limit Florida citrus markets, increase costs of production, and reduce fruit quality as effective chemical controls are not available.
The introduction and establishment of the dreaded Huanglongbing (HLB) disease (citrus greening, yellow dragon disease) caused by species of the phloem-limited bacterial pathogen, Candidatus Liberibacter, to Florida by 2005 has resulted in a 60-70% decline in citrus production and a serious progressive decline in tree vigor and longevity. Without effective bacterial disease control, the 2.68 billion dollar commercial citrus industry in Florida is jeopardized. The vector is present in California and other citrus producing states thus making it highly probable that this disease will soon be present throughout the United States. The lag time from infection to symptom expression for this disease varies from six months to five years depending on age of tree, vigor, and environmental factors (Bove, 2006). This lag in symptom expression provides ample time for infection before detection and containment in a new area can be accomplished.
Candidatus Liberibacter species infect many plant species and plug the plant's vascular (phloem) tissues to limit nutrient movement. Symptoms of this disease reflect a severe deficiency of essential mineral nutrients (for example copper, manganese, zinc). A temporary masking of symptoms can be achieved by applying high rates of foliar nutrients; however, the bacterial pathogen remains active and infected trees continue to decline in over-all vigor and productivity. Antibiotics injected into the tree's vascular system are toxic to the tree, and previously available surface—applied copper compounds are not mobile enough to inhibit bacterial activity within vascular (xylem and phloem) or other plant tissues (parenchyma, mesophyll, etc.). Current HLB control strategies of frequent insecticide sprays to limit populations of the psyllid insect vector, removal of infected trees, and nutrient maintenance to keep existing trees as productive as possible until they die provide little confidence for a sustainable citrus industry or incentive to reestablish it (Bove, 2006; UF/IFAS SWFREC, IMMOKALEE IRREC Seminar, 5 Jun., 2009).
Illustrative of the seriousness of the situation, the Florida Citrus Commission, through the Florida Citrus Advanced Technology Program (FCPRAC), has funded over $18.3 million in research the past two years to develop controls for HLB, and has announced additional funding for this year. Productive citrus acreage in Florida has declined from 1.3 million acres in 2000 to less than 500 thousand acres since the introduction of HLB, and is declining rapidly in the absence of an effective control for HLB. Few growers are willing to risk the large capital costs necessary to reestablish groves decimated by HLB until an effective disease control is available.
Another serious bacterial disease of citrus is Citrus Vareigated Chlorosis (CVC) caused by the xylem-limited Xylella fastidiosa bacterium. In contrast to Ca. Liberibacter species that inhabit the vascular phloem tissues, this bacterial pathogen causes a serious “decline, scorch, or dwarfing” disease of many other perennial fruit, nut, and forage crops by plugging the vascular xylem elements to induce a severe nutrient deficiency leading to plant decline and death.
Micronutrients inhibit, stimulate, and regulate critical physiological processes for plant health and disease control (Datnoff et al., 2009; Huber, 1980; Huber and Graham, 1999; Johal and Huber, 2009). An example is the activation of plant resistance mechanisms by providing a nutrient sufficiency of manganese and copper at the infection site for plant cell division and the production of microbial inhibiting compounds that limit pathogen damage (Huber and Graham, 1999; Johal and Huber, 2009).
The long-standing recognition of the biocidal effects of copper on microorganisms has sometimes overlooked the essential role of copper in plant physiological processes that influence disease resistance. Much of the control attributed to direct microbial toxicity may actually be through increased plant resistance since there often is little correlation between bacterial pathogen population and disease control by copper. Copper is a regulator, component, or co-factor in various enzyme systems involved in plant resistance to disease such as microbial inhibitory flavonoids, lignin, phenols, peroxides, pathogenesis proteins, etc. (Evans et al., 2007). Thus, copper activated physiological processes can increase plant resistance to various bacteria and other pathogens. The requirement of copper in photosynthesis and production of carbohydrates, lignification of vascular tissues for water and nutrient transport, hormone production, amino acid and protein metabolism, and reproduction can have indirect effects on bacterial diseases through alteration of the localized environment to one less conducive for growth, pathogenesis or virulence of the bacterial pathogen. Lignin monomers produced during copper activated lignification have microbiocidal activity as does peroxide generated by copper activated plant oxidases.
Various copper compounds are used for bacterial disease control in production agriculture; however, currently available copper sources (for example, sulfates or oxides) have limited benefit and have not been effective against bacterial pathogens in plant vascular systems (xylem or phloem) because of their limited distribution in plant tissues and interaction with physiological processes. Several bacterial plant pathogens have developed strains that are resistant (tolerant) to the inhibitory effects of copper. Acid phosphorus (phosphorous acid, —PO3) has been used as a fungicide (U.S. Pat. No. 4,075,324), and metal phosphites may provide synergistic activity with several organic fungicides (U.S. patent application No. 2009/0030053), but these have not been developed for plant-associated bacteria and endophytic microorganisms. One of the problems associated with copper is that its mobility in plant tissues is severely restricted by many plant pathogens such that a localized deficiency around infection sites can develop.
Through cooperative research over several years in Brazil with Dr. T. Yamada, an effective control of the xylem-limited bacterium causing CVC was developed and is now extensively used in Brazil. This control involved changing the weed-control management strategy to provide full nutrient sufficiency to the tree. After one to three years after this change in weed-control and nutrient strategy (mulch system to control weeds, inhibit nitrification, and stimulate manganese reducing organisms in the soil), the disease goes into remission and full productivity is restored (Johal and Huber, 2009).
This disease control system, although highly successful for controlling the xylem-limited Xyllella fastidiosa bacterial pathogen, was not effective against the phloem-limited Ca. Liberibacter species causing the HLB or the more tissue-limited foliar Xanthomonas citri bacterium causing the Citrus Canker Disease.